Method of and means for increasing the electrical strength of cables



March 13, 1934. C. F. HANSON METHOD OF AND MEANS FOR INCREASING THE ELECTRICAL STRENGTH OF CABLES Filed May 4, 1928 lllldq.

IN V EN TOR.

A TTORNEYS.

Patented Mar. I3, 1934 METHOD F AND MEANS FOB INCREASING THEs ELECTRICAL STRENGTH OF CA- d BLE Carl F. Hanson, Summit, N. J., assigner to vHabir- 'shaw Cable & Wire Corporation, New York,

N. Y., a corporation of New York Application May 4, 1928, Serial No. 274,982

9 Claims. (Cl. 173-353) Thisinvention relates to electric cables. More specifically it relates to metallically sheathed insulated cables and has for its purpose, among other objects and advantages, to provide a method of and apparatus for increasing the breakdown potential of such cables. l

In testing, splicing, and/or installing, electric cable of the type `referred t it is common to stripaway a portion of the sheath thus exposing a part or length of the insulated conductor. When a voltage is applied across the conductor and the sheath, the electric stresses within the sheathed portion of the cable are more or less evenly distributed, both longitudinally and circumferentially, but at the unsheathed portion or portions the stresses concentrate in the vicinity of the sheath and diminish in intensity as one proceedsl therefrom along the unsheathed conductor. This concentration of stresses at or near the end of the sheath often leads to breakdow-n in this regionor these regions, at a voltage or potential difference between conductor and sheath much lower than that at which breakdown would occur between the sheathed portion of the conductor and its surrounding sheath. In otherwords, the sheathed portion of the cable is able successfully to withstand diierences of potential between conductor and sheath greater than the diierence of potential at which breakdown occurs between the end of the sheath and the unsheathed conductor. It therefore is desirable, both in cable testing and after the cable has been installed that this otherwise occurring concentration of stresses be avoided, i. e. that the stresses be distributed andcontrolled so as to insure that break-down shall not occur at an unsheathed portion of a cable below the breakdown -voltage of the sheathed portion of the cable.

My method of attaining the desired object, and which has other advantages which will be apparent to those skilled in the art after reading the following disclosure, may best be explained in conjunction with disclosure of what l now consider a preferred form of apparatus for carrying the method into practice. Referring to the drawing which illustrates such apparatus-- Fig. 1 is a longitudinal sectional elevation showing the terminal of a cable with my invention applied thereto.

Fig. 2 is a sectional elevation of a so-called pcthead embodying the invention.

Referring'to Fig.` 1 `it will be seen that the cable there illustrated comprises the conductor 10, commonly consisting of a helically twisted group of copper wires, surrounded by the insulation 11, commonly consisting of oil-impregnated paper, in turn surrounded by the metallic sheath 12, commonly consisting of a lead sleeve, tube, or envelope, of which a portion has been stripped away, exposing `a portion of the insulated conductor. In the absence of furtherv provisions, the conductor 10 and sheath 12 constitute two plates of a condenser and, as explained, break-down under increasing potentials or E. M. F.s applied across the conductor 10 and sheath 12 is most likely to occur at or adjacent the end of the sheath. According to my invention the end portion of the cable-sheath is relieved, to the desired extent, of this function by capacitively by-passing the capacity between the conductor and sheath. 'I'he preferred arrangement illustrated is constructed substantially as follows.

A plurality of longitudinally spaced metallic members 14, l5, 16, 17, graded in size are placed around the unsheathed but insulated portion of the conductor 10. Each of .these members 14, 15, 16, 17, consists of a continuous band of metal, such as lead foil or copper, the bands being of increasing-size, the band 17 most remote from the end of the sheath 12 being the largest. Each of these bands constitutes one plate of a plurality of condensers of which the other and common plate is the conductor 10. A similar series of metallic members or -bands 18, 19, 20, 21 are disposed around the sheathed portion of the conductor 10, as shown'. The bands 14 and 18 are connected by a metallic wire or strip 22, the bands l5 and 19 are connected by a metallic wire or strip 23, the bands 16 and 20 are connected by a metallic wire orstrip 24, and the bands 17 and 21 are connected by a metallic wire or strip 25; but except for these connections the bands 14 to 21 are insulated from each other and are also insulated from the cable sheath 12 and conductor 10. The applied-insulation, 26, suppleinenting the insulation 11, may consist of varnished cambric wound or wrapped in tape form around the conductor and cable and in which the bands are embedded.

As stated, each of the bands 14 to 17 constitutes a plate of a condenser of which the conductor l() is the other and common plate. `Likewise each of the bands 18 to 21 constitutes a plate of a condenser but ci which the sheath 12 is the other and common plate. Each of the condensers ifi- 3.0, liv-l0, 16-10, and 17-10, is serially electrically connected through a corresponding one ofY the metallic wires or strips 2 2, 23, 24, 'and 25, to a corresponding one of the condensers 18-12, 19-12, 20-12, and 21-12.

` By providing the proper number of bands, i. e. condensers, of the proper relative size, and by adjusting these bands longitudinally with respect to each other and with respect to the end of the sheath, for the particular cable employed, a

. practically uniform or other desired potential gradient from the unsheathed conductor 10 to the sheath 12 is obtained. Thus the rings 14, 15, 16, and 17,`may be caused to assume and be maintained at any of a number of desired` relative potentials. By virtue of these provisions, the break-'down potential between the unsheathed portion of the conductor 10 and the sheath 12 is no less than that at which break-down will occur between the sheath 12 and the sheathed portion or portions of the conductor 10. Cable may, with these or equivalent provisions, be satisfactorily tested by increasing the diierence of potential applied across the conductor 10 and sheath 12 (by any suitable -source of E. M. F., not

shown) until break-down occurs between the conductor 10 and its sheath 12 at some point or points other than at the unsheathed region.

The invention is not only of advantage in cable testing but also in ordinary service after instal lation, it being important and desirable that break-down shall not occur at any cable terminal at a voltage'below that which the cable is otherwise capable of withstanding. Thus in splicing cable, of the type referred to, it is necessary to unsheath (i. e., strip away the sheath from) the ends of the conductors to be connected. 'Ihe invention, substantially as shown in Fig. 1, may be applied between each suchlunsheathed conductor and its adjacent sheathed portion after the conductors have been joined and before the lead sleeve is applied by joint-wiping in the usual manner. The invention may also be embodied in other cable terminals, for example, in pothead terminals. Such a terminal with the invention applied thereto is shown in Fig. 2 which I shall now proceed to describe.

This pothead comprises a sleeve 30 of the petticoat type, of porcelain or other suitable material, which seats at its lower end in a sleeve or bracket 31 of non-magnetic metal, a gasket 32 and cement 33 serving to provide a fluid-tight seal between the sleeve 30 and the sleeve or bracket 31. At the lower end of the latter is provided a wiping sleeve 34 connected with the sheath of the metallically sheathed insulated conductor by the usual wiped joint 35. A gasket 36 is shown provided' between-the sleeve 31 and the wiping.

sleeve 34 so as to make a fluid-tight joint. The upper end of the pothead shown comprises a hood 40 secured to the sleeve 30 by lmeans of cement 41 and provided with filling and vent holes lneed no further description either as to construction or function. f

The pothead, when completed, may be 4iilled -to the desired level with oil or other suitable insulating fiuid or compound.

In accordance with the provisions ofthe patent f statutes, I have herein described the principle of operation of my invention, together with the apparatus which I now consider to represent the best embodiments thereof, but I desire to have it understood that the apparatus disclosed is only illustrative and that the invention can be carried out by other means. Also, while it is designed to use the various features and elements in the combinations and relations described, `some of these may be altered and others omitted and some of the features of each modification may be embodied in the others without interfering with the more general results outlined, and the invention extends to such use.

What I claim is:-

1. The method of increasing the break-down potential between an` unsheathed portion of a metallically sheathed insulated conductor and the sheath which comprises establishing a plurality of parallel graded capacitive shunt paths across the unsheathed portion of the conductor and the sheath.

2. The combination with a metallically sheathed insulated conductor having an unsheathed portion cf a plurality of graded metallic members each capacitively associated with the unsheathed portion of the first mentioned conductor, a plurality of graded metallic members each capacitively associated with the sheath of the conductor, and metallic connections between each of the first mentioned members and a corresponding one of the second mentioned metallic members.

3. The combination with a metallically sheathed insulated conductor yhaving an unsheathed portion of a plurality of metallic members each capacitively associated with the unsheathed portion of the first mentioned conductor, a plurality of graded metallic members each capacitively associated with the sheath of the conductor, and metallic connections between each of the first mentioned members and a corresponding one of the second mentioned metallic members.

4.y The combination with a metallically sheathed `insulated conductor having an unsheathed portion of a plurality of graded metallic members each capacitively associated with theunsheathed portion of the first mentioned conductor, a plurality of metallic members each capacitively associated with the sheath of the conductor, and methe sheath comprising in combination with the ,conductor and sheath, a plurality of graded condenser plates surrounding the'unsheathed portion of the conductor and spacer longitudinally, and a plurality of graded condenser plates surrounding the sheath of the conductor and each connected to a corresponding one of the first mentioned condenser plates.

6. The combination with a metallically sheathed insulated conductor having an unsheathed portion of a metallic'member capacitively associated with the unsheathed portion of the first mentioned conductor, a metallic member capacitively associated with the sheath of the conductor and insulated therefrom, and a metallic connection between the metallic members, the direct capacity between the `second mentioned metallic member and the sheath being greater than the direct capacity between the second mentioned metallic member and the rst mentioned insulated conductor.

7. The combination with a metallically sheathed insulated conductor having an unsheathed portion of a metallic member capacitively. associated with the unsheathed portion of the rst mentioned conductor, a metallic member capacitively associated with the sheath of the conductor and insulated therefrom, and a metallic connection between the metallic members, the sheath of the rst mentioned conductor being between the second mentioned metallic member and the first mentioned conductor.

8. The combination with a metallically sheathed insulated conductor having an unsheathed portion of a metallic member capacitively associated with the unsheathed portion of the rst mentioned conductor, a metallic member capacitively associated with the sheath of the conductor and insulated therefrom, and a metallic connection between the metallic members, the said second mentioned metallic member surrounding the rst mentioned conductor at its metallically sheathed portion.

9. The combination with a metallically sheathed insulated conductor having an unsheathed portion of a metallic member capacitively associated with the unsheathed portion of the-rst mentioned conductor, a metallic member capacitively associated with the sheath of the conductor and insulated therefrom, and a metallic connection between the metallic members, the end of the metallic sheath being between the said first and second mentioned metallic members.

CARL F. HANSON.

CERTIHCATE or coRREcrxoN.-

Patent No. l, 950, 698.

March 13, 1934;

CARL r. HANsoN.

. It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, line 136, claim 5, for "spacer" read spaced; and that the said Letters Patent should be read with `this correction therein that the same may conform to the record of the case in the` Patent Office.

Signed and sealed this 15th day of May, A. D. 1934.

(Seal) Bryan M. Battey Acting Commissioner of Parents. 

